A fuel cell has characteristics that it is high in efficiency because it can take out electric energy directly from free energy changes caused by combustion of fuel. Furthermore, the fuel cell does not discharge any harmful substance and thus have been extended to be used for various purposes. In particular, a polymer electrolyte fuel cell has characteristics that it is high in power density and compact in size and operates at low temperatures.
A fuel gas for a fuel cell generally contains hydrogen as the main component. Examples of the raw material of the fuel gas include hydrocarbons such as natural gas, LPG, naphtha, and kerosene; alcohols such as methanol and ethanol; and ethers such as dimethyl ether. However, carbon atoms in addition to hydrogen atoms are present in the aforesaid raw materials and thus carbon origin impurities can not be avoided from mixing in the fuel gas to be supplied to a fuel cell. Carbon monoxide in particular poisons a platinum group metal used as an electrocatalyst of a fuel cell. If carbon monoxide is present in a fuel gas, the fuel cell would not be able to obtain sufficient power-generating characteristics. In particular, the lower the operating temperature of a fuel cell is, the more carbon monoxide adsorb to a noble metal catalyst, and the more likely the catalyst is poisoned. It is, therefore, indispensable to decrease the concentration of carbon monoxide in the fuel gas for a system using a polymer electrolyte fuel cell.
A typical example of the method for reducing the carbon monoxide concentration in fuel gas is a method, so-called “water-gas-shift reaction” wherein carbon monoxide in a reformed gas produced by reforming a raw material is allowed to react with steam to convert it to hydrogen and carbon dioxide. However, this method can normally reduce the carbon monoxide concentration only down to 0.5 to 1 percent by volume. The carbon monoxide concentration having been reduced to 0.5 to 1 percent by volume by the water-gas-shift reaction is, therefore, required to be further reduced.
A typical example of a method for further reducing the carbon monoxide concentration includes a method wherein molecular oxygen-containing gas is added to gas containing hydrogen and carbon monoxide to oxidize preferentially and convert the carbon monoxide to carbon dioxide or to methanate the carbon monoxide. Alternatively, a two-step method has been proposed, in which carbon monoxide is methanated at the first step and then oxidized at the second step (see Patent Literature 1 below).
However, mere methanation of carbon monoxide causes the loss of hydrogen that will be used as fuel for a fuel cell and thus is not appropriate in view of efficiency. The aforesaid two-step method can not avoid the loss of hydrogen at the first step, either. It is, therefore, adequate to employ a method wherein carbon monoxide is converted to carbon dioxide by preferential oxidation. The key point of this method is how a trace or small amount of carbon monoxide present in an enormously excess amount of hydrogen is preferentially oxidized to be reduced to a level that is preferable for a noble metal catalyst used in a fuel cell. As the result of a recent demand to improve the performance and durability of a fuel cell, the carbon monoxide concentration of a fuel gas has been required to be 5 ppm by volume or less. In contrary to this, it is difficult to reduce the carbon monoxide concentration to the aforesaid level or lower only by oxidation, and it is preferable to methanate carbon monoxide generated upon oxidation. That is, a catalyst is enhanced in not only preferential oxidation activity but also methanation activity with respect to carbon monoxide so that a trace amount of carbon monoxide remaining unreacted is removed by methanation. In this case, the loss of hydrogen due to methanation is little and does not cause any serious problem in efficiency.
A catalyst loading ruthenium or ruthenium and platinum on an inorganic support is known as a catalyst for use in preferential oxidation of carbon monoxide, and a study has been made, which has reported that the performances of the catalyst can be improved by adjusting the distribution of ruthenium and platinum in the support particles. For example, Patent Literature 2 discloses a catalyst wherein 50 percent or more of the value of integral of the total ruthenium concentration distribution is present in a region of ⅓ of the depth in the radius direction from the support particle surface. However, it does not disclose that platinum is further loaded on the catalyst. A catalyst loading only ruthenium but not platinum has a problem that it is difficult to reduce the carbon monoxide concentration to a requisite level (for example, 10 ppm by volume or less) when the molar ratio of molecular oxygen to carbon monoxide (O2/CO) is within a wide range. A catalyst having ruthenium localized on the outer surface of the support particle tends to be reduced in activity with time because the ruthenium is likely to agglomerate though it has a high initial activity.
Patent Literature 3 discloses a catalyst wherein ruthenium and platinum are localized within the range of 100 μm inwardly from the outer surface of an α-alumina support particle. However, when carbon monoxide in hydrogen is preferentially oxidized using such a catalyst, the concentration of the remaining carbon monoxide is reduced to the order of only several tens ppm by volume. This is assumed to be related to the use of the α-alumina support with a relatively small specific surface area. All of these conventional techniques intend to improve an activity for preferential carbon monoxide oxidation by localizing ruthenium (and platinum) in the vicinity of the surface of the support particle at a higher level.
The inventors of the present invention have filed a patent application for a catalyst comprising ruthenium in a specific distribution in the support particle and also loading platinum (see Patent Literature 4). However, a further improvement in catalyst performances has been demanded in conformity with the requests for the improved performances and prolonging the working life of a fuel cell system.